1. Field of the Invention
The present invention relates to electromagnetically operated hydraulic valves, and, more particularly, to an adjustment unit for a hydraulic control valve, especially a proportional-response valve which serves for the continuous adjustment of either the pressure or the flow rate in conjunction with a hydraulic control circuit.
2. Description of the Prior Art
It is known to use electromagnetically controlled proportional-response control valves in hydraulic circuits, for the continuous adjustment of pressure values or flow rate values, either in response to a randomly preset input value, or in response to a changing input value, as in the case of a feedback loop arrangement.
The adjustment unit for such a proportional-response control valve includes a proportional-response electromagnet of the solenoid-type, with an armature which is guided for axial movements against a return spring. This proportional-response magnet unit is normally connected end-to-end to the housing of the control valve.
It is also known to extend the hydraulic space of the control valve into the magnet unit, so that the space which is occupied by the armature of the latter is filled with hydraulic fluid, thereby eliminating friction-producing seals between the control valve and the armature of the magnet unit.
Also known from the prior art is the arrangement of an inductive displacement transducer in conjunction with the proportional-response magnet unit, the displacement transducer consisting of a permanent magnet which is mounted on an axial extension of the push rod of the magnet armature and a stationary coil surrounding the transducer magnet. The coil is carried by a housing which is axially aligned with and attached to the housing of the magnet unit. Such an arrangement is disclosed in "o+p Olhydraulik und Pneumatik", Vol. 21, No. 10, 1977, p. 722 ff. and in a paper by H. Walter in the periodical "fluid", Oct. 1978, pp. 28-34.
In this prior art device, the fluid-filled space around the armature of the proportional-response magnet is sealed off from the space which contains the inductive displacement transducer. This requires the presence of a sealing element which cooperates with the moving armature under axial friction. As a result of this friction, it has been found that this arrangement is subject to erratic movements, known as "stickslip" action, thereby distorting the axial displacements of the armature and the accuracy of operation of the control valve. An additional shortcoming of the above-described prior art device is the potential for misalignments in the mounting relationship between the coil of the inductive displacement transducer and the transducer magnet.